Proton conducting materials containing solid acids have been widely used. For example, Non-Patent Literature 1 describes that CsHSO4 and CsH2PO4, each of which is a solid acid, become favorable proton conductors at 230° C. or higher. Non-Patent Literature 2 describes that crystalline Sr1-xInxP2O7 exhibits a very high proton conductivity around 200° C. Each of these substances, however, has a problem of a poor membrane-forming property. If there is an inorganic proton conducting material, i.e., a proton conducting membrane having a fine membrane-forming property, this will further expand the range of choices for a proton conducting membrane depending on the purpose and/or the like.
[Non-Patent Literature 1]    S. M. Haile et al. Nature 410, 910 (2001)
[Non-Patent Literature 2]    M. Nagao, et al., J. Electrochem. Soc., 153, A16004 (2006)